Battery systems that employ a plurality of interconnected batteries have been designed to address the need for battery systems that can power the movement of vehicles such as cars. Many of these systems connect batteries in series. However, different batteries in a series will have different capacities, different self-discharge rates and/or different impedances that will affect the performance of these systems. Drawing a high current from these systems can cause the voltage of one or more batteries to fall to a dangerously low level and fail while the remaining batteries remain operational. The failure of the battery can cause failure of the entire system. There is a need for an improved battery system that addresses the variations in the performance of different batteries.